Photolithography is a key process in the fabrication of semiconductor integrated circuits and many optical, magnetic and micromechanical devices. Lithography creates a pattern on a thin film carried on a substrate so that, in subsequent process steps, the pattern can be replicated in the substrate or in another material which is added onto the substrate. Conventional lithography typically involves applying a thin film of resist to a substrate, exposing the resist to a desired pattern of radiation, and developing the exposed film to produce a physical pattern. In this approach, resolution is limited by the wavelength of the radiation, and the equipment becomes increasingly expensive as the feature size becomes smaller.
Imprint lithography, based on a fundamentally different principle, offers high resolution, high throughput, low cost and the potential of large area coverage. In imprint lithography, a mold with microscale or nanoscale features is pressed into a thin film, deforming the shape of the film according to the features of the mold and forming a relief pattern in the film. After the mold is removed, the thin film can be processed to remove the reduced thickness portions. This removal exposes the underlying substrate for further processing. Details of imprint lithography are described in applicant's U.S. Pat. No. 5,772,905 issued Jun. 30, 1998 and entitled “Nanoimprint Lithography”. The '905 patent is incorporated herein by reference.
The usual method of pressing the mold into the thin film involves positioning the mold and the substrate on respective rigid plates of a high precision mechanical press. With such apparatus, the process can generate sub-25 nm features with a high degree of uniformity over areas on the order of 12 in2. Larger areas of uniformity would be highly advantageous to increase throughput and for many applications such as displays.
The use of a high precision mechanical press to press a mold into a thin film presents tolerance problems in replicating small patterns over large areas. Presses move on guide shafts through apertures, and the spacings between the shafts and their respective apertures can be large compared to the features to be replicated. Such spacings permit undesirable relative translational and rotational shifts between the substrate and the mold. Moreover, despite the most careful construction, the molds and the substrates used in lithography are not perfectly planar. When these molds and substrates are disposed on the rigid plates of a press, the deviations from planarity over large areas can result in variations in the molding pressure and depth of imprint. Accordingly, it is desirable to provide a method of imprint lithography which avoids the limitations of mechanical presses.
An alternative method of pressing the mold into the thin film is the technique of fluid pressure imprint lithography described in applicant's U.S. Pat. No. 6,482,742 issued Nov. 19, 2002 and entitled “Fluid Pressure Imprint Lithography”. In this method the molding surface is disposed adjacent the film, the molding surface/film interface is sealed and pressurized fluid is used to force the molding surface into the film. Since the pressure is isostatic, translational and rotational shifts are minimal, and smaller features can be imprinted with high uniformity over larger areas than can be imprinted using mechanical presses.
Fluid pressure imprinting has dramatically improved nanoimprint lithography. A further improvement for commercial manufacture would be a method which could provide comparable results without the necessity of sealing the molding surface/film interface.